1. Field of the Invention
The present invention relates to a high voltage porcelain insulator. More particularly, the present invention relates to a high voltage porcelain insulator which is suitably used for direct current (DC) high voltage use but which is also available to alternating current (AC) high voltage use.
2. Description of the Prior Art
Heretofore, high voltage porcelain insulators have been used for electric precipitators or electric power systems, when they are used for DC high voltage line. However, they are liable to deteriorate at their porcelain portions, especially when they are used at high electrical field concentration or at high temperatures.
As porcelain materials porcelains of DC insulators for use at high temperatures, hitherto, use has been made of porcelains which have high specific resistivity, superior thermal shock property and low thermal expansion coefficient.
As various studies on deterioration properties of porcelains due to energization of DC voltage, there are such reports as Kh. S. Valeyev et al "The Variation of the Electrical Strength of High Voltage Porcelain under Sustained High Direct Voltage" Elektrichestvo, No. 1, pp. 59-61 (1963) and P. P. Budnikov et al "The Effect of Vitreous Phase on the Properties of High-Tension Electrical Porcelain" Xth International Ceramic Congress in Stockholm, Sweden, June 12-18, 1966. In these reports, deterioration properties of material porcelains of various compositions due to energization of DC voltage have been evaluated by using test piece specimens of the porcelains and it is concluded that specific resistivity of porcelains should preferably be made high in order to decrease the deterioration phenomena of the porcelains due to energization of DC voltage. The conclusion has been explained by the reason that when a porcelain is applied with DC high voltage at high temperatures, ionic current is produced in the porcelain and a layer of high specific resistivity which contains a decreased amount of alkali metal component is formed at the positive electrode of the porcelain and a deposit of alkali metal is formed at the negative electrode of the porcelain and hence mechanical properties and electrical properties of the porcelain are deteriorated.
According to results of tests on test pieces which the inventors had performed for evaluating the deterioration property of porcelains under energization of DC voltage, a tendency of a porcelain has been confirmed that rate of decrease of dielectric breakdown voltage becomes low as specific resistivity of the porcelain becomes high as shown in the attached FIG. 1, similarly as in the results of the tests of the above known publications. This is presumably due to a reason that, under a certain condition of energization of DC voltage, a porcelain of a high specific resistivity allows small electric current to flow and hence has smaller effect in varying the material porcelain.
Though there have been conducted studies on deterioration of porcelains due to energization of DC voltage using test piece specimens as mentioned above, there are very few studies on deterioration properties of porcelain insulators due to energization of DC voltage, which were produced by assembling porcelains and hardwares with the aid of cement and the like. Therefore, the mechanism of deterioration of the porcelain insulators due to energization of DC voltage has not yet been clarified.